Direct and Indirect Effects of Increased CO2 Partial Pressure on the Bioenergetics of Syntrophic Propionate and Butyrate Conversion

Simultaneous digestion and in situ biogas upgrading in high-pressure bioreactors will result in elevated CO 2 partial pressure (pCO 2 ). With the concomitant increase in dissolved CO 2 , microbial conversion processes may be affected beyond the impact of increased acidity. Elevated pCO 2 was reported to affect the kinetics and thermodynamics of biochemical conversions because CO 2 is an intermediate and end-product of the digestion process and modifies the carbonate equilibrium. Our results showed that increasing pCO 2 from 0.3 to 8 bar in lab-scale batch reactors decreased the maximum substrate utilization rate ( r smax ) for both syntrophic propionate and butyrate oxidation. These kinetic limitations are linked to an increased overall Gibbs free energy change (Δ G Overall ) and a potential biochemical energy redistribution among syntrophic partners, which showed interdependence with hydrogen partial pressure (pH 2 ). The bioenergetics analysis identified a moderate, direct impact of elevated pCO 2 on propionate oxidation and a pH-mediated effect on butyrate oxidation. These constraints, combined with physiological limitations on growth exerted by increased acidity and inhibition due to higher concentrations of undissociated volatile fatty acids, help to explain the observed phenomena. Overall, this investigation sheds light on the role of elevated pCO 2 in delicate biochemical syntrophic conversions by connecting kinetic, bioenergetic, and physiological effects.